The aim of the present study was to determine the portal-drained viscera (PDV) heat production (HP) and net PDV flux of volatile fatty acids (VFA: acetic, propionic and butyric acids) in Iberian pigs (34-kg bodyweight) fed with acorn from evergreen oak, and to ascertain whether there is an effect of acorn feeding over time. In addition, potential contribution of both parameters to the energy budget of the animal was calculated. The following two sampling periods were conducted with six gilts: after 1 day (I) and after 1 week (II) of acorn feeding. Postprandial PDV HP was 29% greater (P < 0.01) in Sampling period II than in Sampling period I, but net PDV flux of VFA was only slightly greater (5%, P > 0.1). Potential proportional contribution of VFA to the whole HP was almost identical in both sampling periods (0.057, on average), representing 0.109 (on average) of the metabolisable energy requirements for maintenance. Pigs adapted for 1 week to an acorn diet had increased PDV HP without an increase in the net PDV flux of VFA, indicating that, apparently, nutrients other than VFA were responsible for the increased PDV HP.